The present invention relates to a shift control device for an automatic transmission of the electronic control type, and more particularly, to a shift control device incorporating an automatic shift pattern changeover means therein.
The shift pattern of an automatic transmission for an automotive vehicle or the like is primarily determined to be the most suitable pattern by every vehicle by assuming a certain running condition (typically surface gradient or grade of road surface). The running condition of vehicle, however, is successively changed and particularly this change is most considerable on mountainous roads so that the above shift pattern is not always the most suitable or comfortable.
This shift pattern determines a shift-up line so as to select the highest speed capable of accelerating the vehicle, and determines a shiftdown line by shifting it slightly to the low speed side than of the shift-up line, based on a relationship between the driving torque of vehicle caused by transmitting an engine output torque changed in accordance with engine load (for example, the degree of throttle opening) to a driving wheel through a reduction gear, and a running resistance under the certain running conditions.
The acceleration of a vehicle obtained from a relation between the driving torque and the running resistance of a vehicle, here, is considered. The running resistance is changed as shown in FIG. 1 by a, b and c when, for example, the grade of road surface is 0%, 5% and 10%, respectively. Each running resistance a, b, c becomes large as a vehicle speed becomes high since air resistance increases and the difference between running resistances a, b and c is the difference in the running resistance caused by the difference in the grade of the road surface. The driving torque differs by the load condition of engine, for example, the degree of throttle opening and the reduction gear ratio (speed change or shift position). For example, when the throttle opening degree is 1/8, 2/8 and 7/8 in the case of vehicle having two speed shifts the driving torque is changed as, for example, shown in FIG. 1 by curves d, e and f, respectively. In FIG. 1 the lower speed side (left side in the drawing) the vehicle speeds V.sub.d, V.sub.e, V.sub.f of each driving torque line d, e, f shows the case of selecting the low speed shift, while the high speed side (right side in the drawing) shows the case of selecting the high speed shift.
In FIG. 1, let the vehicle speed V be V.sub.1, a running resistance L is L.sub.a, L.sub.b and L.sub.c when the grade of road surface is 0%, 5% and 10%, respectively, and a driving torque T becomes T.sub.f when running at a high speed shift with the throttle opening of 7/8. Therefore, during these running conditions, if the grade of road surface is 10%, 5% and 0%, respectively, torque allowance .DELTA.T becomes as shown by .DELTA.T.sub.1, .DELTA.T.sub.2 and .DELTA.T.sub.3, respectively, and further becomes smaller as the grade of road surface becomes sharp. An acceleration .alpha. given to a vehicle becomes large in proportion to the torque allowance, and is expressed by .alpha.=(.DELTA.T/K) (K is a constant corresponding to inertial mass of the vehicle). The driving torque T varies according to the throttle opening, so that if the throttle opening is changed from 7/8 to 2/8, the torque is lowered from T.sub.f to T.sub.e. In this case, if a vehicle is running on the road having the road surface gradient of 10%, the driving torque T.sub.e is smaller than the running resistance L.sub.c, so that the vehicle cannot be accelerated.
With the aid of the above relation, from driving torque curves (d-f) and running resistance curves (a-c) determined for every vehicle, the present throttle opening, vehicle speed and shift position are taken into consideration, so that a reference acceleration obtained during this running can be obtained, the thus obtained reference acceleration is compared with the actual vehicle acceleration so as to judge the road gradient, shift patterns D, S.sub.1 and S.sub.2, where the sharper the road surface gradient as shown in FIGS. 2a, 2b and 2c, the higher the shift line, are selected, and whatever the road surface gradient is, smooth and efficient acceleration can be carried out, thereby solving a problem inherent to the shift control device having a single shift pattern. Such device has been proposed.
However, when the vehicle is running on the road surface in which gradient is often changed, with the use of the above automatic transmission, a shift pattern is often changed by every gradient change in a short time, unnecessary up-shift and down-shift are frequently carried out, so that riders receive shocks every time and feel uncomfortable.
Therefore, there has been such ideas that (1) in case only when a certain time is required from a decision for changing a shift pattern to the next decision for change, the shift pattern is actually altered to the corresponding one, and (2) a decision for changing a shift pattern is made by every certain time and in case only when an instruction is given for selecting another speed change pattern, the shift pattern is altered.
Even if either one of the above ideas (1) and (2) is employed, if the certain time is prolonged, the change of shift pattern in response to the change of the road surface gradient is largely delayed. That is, when the vehicle enters a steep ascent from a flat road, the change from the D pattern to an S.sub.1 or S.sub.2 pattern is largely delayed, so that the running at the high speed shift is forced during this delay period thereby to prevent smooth acceleration, while if the vehicle enters from a steep ascent to a flat road, the change from the S.sub.1 or S.sub.2 pattern to the D pattern is delayed by a large margin, the running at the low speed shift is forced during this delay period, thereby to prevent effective utilization of the engine output, so that it is impossible to avoid aggravation of fuel consumption. If the certain time is shortened, the change of the shift pattern becomes unnecessarily frequent as described above, comfortable riding becomes worse, and an attempt for solving this problem cannot be attained.